The 72 year rule refers to the time between the start of a census and when the individual’s responses are available to public for inspection. This specifically includes the population schedule forms, which are most useful to genealogists. The population schedules identify the number of individuals living in a household. Since 1850, each individual by name was recorded. Prior to 1850, only a count of the individuals living in the house hold was taken.
As you can imagine population schedules can prove very useful in determining a family makeup. However, because of the 72-year rule, the population schedules for recent Censuses are not available. The most recent population schedules available to the public from a census are 1940.
Why 72 years? This number is based on the 1952 average life expectancy of a woman (71.6 years) in the United States. In 1978, this average was used to enact privacy legislation regarding the US census into law.